Charged-particle microscopy is a well-known and increasingly important technique for imaging microscopic objects, particularly in the form of electron microscopy. Historically, the basic genus of electron microscope has undergone evolution into a number of well-known apparatus species, such as the Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), and Scanning Transmission Electron Microscope (STEM), and also into various sub-species, such as so-called “dual-beam” tools (e.g. a FIB-SEM), which additionally employ a “machining” Focused Ion Beam (FIB), allowing supportive activities such as ion-beam milling or Ion-Beam-Induced Deposition (IBID), for example. More specifically:                In a SEM, irradiation of a specimen by a scanning electron beam precipitates emanation of “auxiliary” radiation from the specimen, in the form of secondary electrons, backscattered electrons, X-rays and cathodoluminescence (infrared, visible and/or ultraviolet photons), for example; one or more components of this emanating radiation is/are then detected and used for image accumulation purposes (for example).        In a TEM, the electron beam used to irradiate the specimen is chosen to be of a high-enough energy to penetrate the specimen (which, to this end, will generally be thinner than in the case of a SEM specimen); the transmitted electrons emanating from the specimen can then be used to create an image. When such a TEM is operated in scanning mode (thus becoming a STEM), the image in question will be accumulated during a scanning motion of the irradiating electron beam.        
As an alternative to the use of electrons as irradiating beam, charged particle microscopy can also be performed using other species of charged particle. In this respect, the phrase “charged particle” should be broadly interpreted as encompassing electrons, positive ions (e.g. Ga or He ions), negative ions, protons and positrons, for instance.
It should be noted that, in addition to imaging and performing (localized) surface modification (e.g. milling, etching, deposition, etc.), a charged particle microscope may also have other functionalities, such as performing spectroscopy, examining diffractograms, etc.
In all cases, a Charged-Particle Microscope (CPM) will comprise at least the following components:                A particle source, such as a Schottky electron source or ion source.        An illuminator, which serves to manipulate a “raw” radiation beam from the source and perform upon it certain operations such as focusing, aberration mitigation, cropping (with a diaphragm), filtering, etc. It will generally comprise one or more (charged-particle) lenses, and may comprise other types of (particle-)optical component also. If desired, the illuminator can be provided with a deflector system that can be invoked to cause its exit beam to perform a scanning motion across the specimen being investigated.        A specimen holder, on which a specimen under investigation can be held and positioned (e.g. tilted, rotated). If desired, this holder can be moved so as to effect scanning motion of the specimen w.r.t. the beam. In general, such a specimen holder will be connected to a positioning system. When designed to hold cryogenic specimens, the specimen holder will comprise means for maintaining said specimen at cryogenic temperatures, e.g. using an appropriately connected cryogen vat.        A detector (for detecting radiation emanating from an irradiated specimen), which may be unitary or compound/distributed in nature, and which can take many different forms, depending on the radiation being detected. Examples include photodiodes, CMOS detectors, CCD detectors, photovoltaic cells, X-ray detectors (such as Silicon Drift Detectors (SDD) and Si(Li) detectors), etc. In general, a CPM may comprise several different types of detector, selections of which can be invoked in different situations.        
In the particular case of a dual-beam microscope, there will be (at least) two sources/illuminators (particle-optical columns), for producing two different species of charged particle. Commonly, an electron column (arranged vertically) will be used to image the specimen, and an ion column (arranged at an angle) can be used to (concurrently) modify (machine/process) and/or image the specimen, whereby the specimen holder can be positioned in multiple degrees of freedom so as to suitably “present” a surface of the specimen to the employed electron/ion beams.
In the case of a transmission-type microscope (such as a (S)TEM, for example), a CPM will specifically comprise:                An imaging system (imaging particle-optical column), which essentially takes charged particles that are transmitted through a specimen (plane) and directs (focuses) them onto analysis apparatus, such as a detection/imaging device, spectroscopic apparatus (such as an EELS device: EELS=Electron Energy-Loss Spectroscopy), etc. As with the illuminator referred to above, the imaging system may also perform other functions, such as aberration mitigation, cropping, filtering, etc., and it will generally comprise one or more charged-particle lenses and/or other types of particle-optical components.        
A particular application of a charged-particle microscope is in performing X-ray spectroscopy. An example of such spectroscopy is Energy-Dispersive X-ray Spectroscopy, which is often referred to using the acronyms EDX or EDS. In this technique, a specimen (e.g. a mineralogical or semiconductor sample) is bombarded with a focused input beam of electrons, e.g. in a SEM or (S)TEM. A lower-shell electron in an atom of the specimen can be ejected from its orbit by a collision with one of these bombarding electrons, creating an electron hole that is promptly filled by the de-excitation of a higher-shell electron in the atom in question, with the concurrent release of a quantum of energy in the form of an X-ray photon. The energy signature/distribution of photons emitted in this way will be characteristic of the particular electron shell structure of the atom in question, and can thus be used as a “fingerprint” in performing compositional analysis of the specimen. An energy-dispersive spectrometric detector collects, sorts and counts the different photons of different energies, producing a measured spectrum for the location of the specimen onto which the focused input beam was directed; such a spectrum can be rendered as a graph of counts per channel (ordinate) versus channel number (abscissa), corresponding to intensity versus energy, and generally comprising a Bremsstrahlung background and various characteristic peaks—whose energy can be used to identify the generating material (which may be an element, chemical compound or mineral, for example, and which may be amorphous or crystalline in nature, for example) and whose height can (in principle) be used to estimate relative quantity of the generating material. If desired, one can then (automatically) move the specimen and/or the beam so that the beam is directed onto a new location on the specimen, and (automatically) repeat the process described above at said new location. This technique is particularly useful in the field of mineralogy, in which a small specimen may contain many different kinds of minerals; however, its usefulness in fields such as metallurgy, microbiology and semiconductor science is also self-evident.
As here employed, the term EDX encompasses so-called Wavelength Dispersive X-Ray Spectroscopy (WDX or WDS). This latter technique can be regarded as a particular refinement of EDX in which the X-rays emerging from a specimen are filtered (e.g. with the aid of a particular type of crystal), so that only X-rays of a given wavelength are counted at any given time.
Another such spectroscopic technique is Proton-Induced X-Ray Emission (PIXE), in which the input beam comprises protons. PIXE can, for example, be performed in a proton microscope.
In what follows, the techniques disclosed herein may—by way of example—sometimes be set forth in the specific context of electron microscopy; however, such simplification is intended solely for clarity/illustrative purposes, and should not be interpreted as limiting.
A problem with known CPM-based X-ray spectroscopy techniques is that they cannot be satisfactorily used to detect relatively light chemical elements (or compounds) with “low” atomic numbers Z—typically below a threshold value Zo of 5 or 6. This group of “excluded elements” includes extremely important members such as:                Lithium (Z=3): Important in the manufacture of rechargeable batteries, engineering alloys, refractive materials and drugs. It does not occur as a metal in nature, but is found in a variety of minerals in igneous rocks.        Beryllium (Z=4): Used to manufacture engineering allows (particularly for the aviation industry), X-ray transmissive optical elements and neutron reflectors/moderators in nuclear reactors. Beryllium and its compounds are toxic and carcinogenic, so that it can be important to be able to detect trace quantities of them.        
This shortcoming is inter alia attributable to the following:                The so-called “zero peak” (at/near zero spectral energy) tends to overwhelm the low-energy signals from low-Z elements.        Many X-ray detectors employ an X-ray window (e.g. comprised of Be), which tends to absorb low-energy X-rays from light elements. Such windows are, for example, used to prevent a cooled X-ray detector from becoming a cryo-trap in the vacuum environment of a CPM.        
Some researchers claim to be able to reliably detect Boron (Z=5) using low-speed, low-kV acquisition; however, for “normal” (high-speed) acquisition, such detection is essentially impractical, in which case Boron is also often regarded as being a member of the aforementioned “excluded elements”. Boron is used in the manufacture of medicaments and vitreous materials, and as a dopant in the semiconductor industry.
Another issue with known CPM-based X-ray spectroscopy techniques is that they have problems dealing with overlapping doublet lines—where a characteristic line of a first element overlaps with that of a second (or further) element, forming a convoluted hybrid feature that cannot be satisfactorily resolved. In such situations, it is challenging to try to quantify the proportions of each contributing element contributing to the overlapping doublet.